An organic electroluminescent element is a self-light-emitting element and therefore, it is bright, is excellent in visibility, and enables clear display compared with a liquid crystal element, so that active researches have been made on it.
In recent years, as an attempt to increase light emission efficiency of elements, an element caused to emit phosphorescence by making use of a phosphorescent emitter, that is, using light emission from a triplet excited state has been developed. According to the theory on the excited state, using phosphorescence emission enables about 4 times greater emission efficiency than that caused by the conventional fluorescence emission and a marked improvement in emission efficiency can be expected.
In 1993, M. A. Baldo, et al. in Princeton University (US) realized an external quantum efficiency of 8% by making use of a phosphorescent element using an iridium complex.
A phosphorescent emitter causes concentration quenching so that it is supported by doped with a charge transporting compound generally called “host compound”. The phosphorescent emitter to be supported is called a “guest compound”. As the host compound, 4,4′-di(N-carbazolyl)biphenyl (hereinafter abbreviated as “CBP”) represented by the following formula has ordinarily been used (for example, Non-Patent Document 1).

It has however been pointed out that CBP has a glass transition point (Tg) as low as 62° C. and has high crystallinity so that it has poor stability in a thin film state. As a result, in case where heat resistance is required, for example, in high-brightness light emission, satisfactory element properties had not been achieved.
With the progress of research on phosphorescent elements, an energy transfer process between a phosphorescent emitter and a host compound has been elucidated. It has therefore been revealed that in order to heighten an emission efficiency, the excited triplet level of the host compound should be higher than the excited triplet level of the phosphorescent emitter.
When a host compound of a light-emitting layer is obtained by doping FIrpic which is a blue phosphorescent material represented by the following formula with the CBP, the external quantum efficiency of the phosphorescent element remains at about 6%. This was presumed to occur due to insufficient confinement of triplet excitons with FIrpic because the excited triplet level of CBP is as low as 2.57 eV while the excited triplet level of FIrpic is 2.67 eV. This has been proved by that the photoluminescence intensity of a thin film obtained by doping CBP with FIrpic shows temperature dependence (Non-patent Document 2).

As a host compound having an excited triplet level higher than that of CBP, 1,3-bis(carbazol-9-yl)benzene (hereinafter abbreviated as “mCP”) represented by the following formula has been known. This mCP has also a glass transition point (Tg) as low as 55° C. and has high crystallinity so that it has only poor stability in a thin film state. Therefore, in case where heat resistance is required, for example, in high-brightness light emission, satisfactory element properties had not been achieved (Non-patent Document 2).

Further, as a result of investigation on host compounds having a higher excited triplet level, it has been found that when an iridium complex is doped with an electron-transporting or bipolar transporting host compound, high emission efficiency can be achieved (for example, Non-patent Document 3).
Thus, in order to enhance the emission efficiency of a phosphorescent element in practical utilization, there has been a demand for a host compound for a light-emitting layer, which has a high excited triplet level and high thin-film stability.